The ability of a cell to produce and respond to its own growth factors has been referred to as autocrine growth. The events involved are generally viewed as consisting of the secretion of the growth factor followed by its binding back to the appropriate surface receptor on the secreting cell. More recently, limited numbers of autocrine loops have been described in which the growth factor is not secreted, but rather binds to the receptor in the secretory compartment and carries out its stimulatory activity intracellularly. This is referred to as an internal autocrine loop rather than the extracellular autocrine loop described above. Using a well-characterized bank of human colon carcinoma cells we have found that transforming growth factor alpha (TGF-alpha) is an autocrine growth factor. Furthermore, we have identified specific growth regulatory phenotypes which show different autocrine mechanisms in colon cancer. Well-differentiated GEO cells show a classical external autocrine loop while poorly-differentiated HCT 116 cells show an internal autocrine loop. This is the first description of an internal autocrine loop for TGF-alpha, the first description of an internal autocrine loop in a human cell line, and the first "native" internal loop in which the growth factor has not been transfected into the cell line expressing this autocrine mechanism. It is hypothesized that the internal autocrine mechanism results in the loss of cell cycle controls such that the cells are constitutively stimulated and lose the ability to respond to normal controls for growth and differentiation. These hypotheses will be tested by carrying out the following specific aims. (1) Determine the extent to which internal and external loops are expressed in a well-characterized bank of colon cancer cell lines. (2) Determine the effects of uncoupling the internal TGF-alpha loop and restoring response to external controls on the expression of cell cycle related molecules. (3) Create internal TGF-alpha loops in untransformed cells and cancer cells which currently utilize external loops. (4) Determine the effects of carrying out specific aim #3 on growth controls. (5) Determine the relationship between differentiation and internal vs. external loop mechanisms for TGF-alpha.